1. Field of the Invention
The present invention relates to optical illumination systems and in particular to optical illuminator configured from arrays of Vertical Cavity Surface Emitting Lasers (VCSEL).
2. Related Background Art
Optical illumination is widely used in a large number of applications whether applied to reflect incident optical radiation, such as optical imaging, or medical imaging, etc. or applied to absorb incident radiation, such as optical pumping of a gain medium in a solid state laser. New and more efficient optical sources are emerging that may be suitable for applications that require small foot print, higher uniformity, high speed or quasi continuous wave (QCW) illumination, just to name a few. One such class of new optical sources is configured using Vertical Cavity Surface Emitting Lasers (VCSEL) and arrays of VCSELs.
One advantage of VCSELs is that the divergence angle of emitted light is very small. Consequently, emission from VCSELs is highly directional even at large distances from the origin. VCSEL emission in visible and infrared wavelengths may be obtained in a very narrow wavelength band allowing optically efficient filtering of background noise. VCSELs are also amenable to generating short pulses, thereby making these sources suitable for LIDAR type applications and for time domain distance discrimination applications such as seeing through fog and motion and gesture detection, etc.
Current technology for packaging VCSELs is quite complex. In most instances, packages typically have electrical leads that are designed for through hole mounting on electronic circuit boards and are described in many prior art patent and non-patent literature publications. In a co-authored and co-owned pending U.S. patent application Ser. No. 13/337,098 filed on Dec. 24, 2011, by Seurin et al, a surface mount packaging is disclosed, the content of which is hereby incorporated by reference in its entirety. More specifically, a single VCSEL and/or arrays of VCSELs are mounted on a thermal submount and then installed in surface mountable housing. The thermal submount may be bonded to a thermal substrate, or in a housing including external cooling devices, by solder or epoxy. The VCSEL or arrays of VCSELs are electrically connected to the submount and to the package conductor pads, using wire or ribbon bonding.
Similar approach of a surface submount is also disclosed in other patents and patent application publications. For example, in the U.S. Pat. No. 7,777,173 issued to Price et al. on Aug. 10, 2010, in the United States Patent Application Publication No. 2003/0185499 A1 by Butler et al. published on Oct. 2, 2003, and in United States Patent Application Publication No. 2006/0088254 A1 by Mohammed published on Apr. 27, 2006. In some of the submounts described in the cited prior art, additional optical elements are included for controlling beam shape.
The beam divergence of VCSEL is quite small and typically has a Gaussian or pseudo-Gaussian distribution. Depending on the application it is often required to adjust the illumination pattern by either increasing the divergence or by changing the distribution from the Gaussian distribution to approximately a top-hat distribution. In other types of mounting configuration well known in the art, a diffuser to adjust a beam distribution pattern is used. For example, in the U.S. Pat. No. 5,946,121 issued to Jiang et al, on Aug. 31, 1999, use of a diffuser attached to the package lid to increase the divergence of a single emitter in a data link application is described. In the United States Patent Application Publication No. 2008/0310852 by Tan et al, published on Dec. 18, 2008, a diffuser is provided to compensate for misalignment in an optical communications transmitter comprising a single VCSEL emitter.
Diffusers alone, or in combination with other optical components, are used with VCSEL and VCSEL arrays as described in many other publications. A diffuser to reduce the coherence in an array of different colored emitter for electronic imaging is disclosed in a U.S. Pat. No. 6,950,454 issued to Kruschwitz on Sep. 27, 2005. However, no detailed description is provided about how the diffuser is attached in the system especially since the diffuser has to be vibrated to eliminate speckle effects. In the United States Patent Application Publication No. 2008/0079904 by Bartlett, published on Apr. 3, 2008, layouts of VCSEL arrays in combination with diffuser in complex alignment with other optical components, is described to generate a uniform illumination beam.
In a different application described in the U.S. Pat. No. 7,150,552 issued to Weidel on Dec. 19, 2006, a diffuser is placed close to the collection lens to obtain a uniform beam from a VCSEL array. In all of the above examples, the diffuser is an additional optical component bonded to a separate fixture, located a distance away from the laser emitter and requires accurate alignment and positioning.
In other arrangements, a diffuser may be placed in close proximity to the VCSEL or VCSEL array. For example, in the U.S. Pat. No. 7,949,022 issued to Miesak et al on May 24, 2011, a diffuser plate in placed proximal to a VCSEL array for optically pumping a solid state laser. The diffuser is close to the VCSEL array but requires a separate mounting to hold it in place between the VCSEL array and the solid-state laser crystal. In an alternative arrangement, individual diffusers separately bonded to each VCSEL in an array is described in the United States Patent Application Publication No. 2003/0026310 by Valliath, published on Feb. 6, 2003. This approach requires aligning each diffuser individually over the emitter and then bonding it in place. For larger arrays having many emitting devices the assembly procedure becomes very expensive.
In this invention a robust and efficient surface mountable optical illuminator is provided that can be used as a surface mount component or as a stand-alone module. The illuminator comprises VCSEL array(s) bonded to a transparent carrier substrate (carrier substrate hereinafter) which seals the emission surface and also provides a robust support. One advantage of the surface mountable packaging option is to avoid wire or ribbon bonding of VCSEL arrays that may become a cause of device failure over long run and that increase module costs. The emitting or non-emitting surface of the VCSEL or VCSEL arrays may be integrated with electronic circuit(s) to electrically connect illuminator module to a drive and/or control circuit(s) on a circuit board using standard surface mounting methods.
The carrier substrate is designed to optionally include optical components such as microlenses, diffusers, etc. The VCSEL array with the carrier substrate may be adapted for surface mounting on a circuit board or a submount as the application demands. The optical illuminator as disclosed in this invention does not require intricate alignment of optical components with individual VCSELs in the array, thereby is readily adaptable for high throughput manufacturing.